888 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



its membranes. The outermost of the foetal membranes is the chorion ; 

 this is covered with vascular villi, which dip into the decidua capsu- 

 laris and basalis. Inside the chorion is the amnion, a closed sac, which 

 surrounds the embryo and is attached to its ventral wall at the 

 umbilicus. The amnion is filled with fluid, the amniotic fluid in 

 which the foetus floats, and it forms a sheath for the umbilical cord by 

 which after a certain time, the foetus is attached to the inner surface 

 of the chorion, or outer embryonic membrane. The umbilical cord 



contains not only the blood- 

 vessels which pass between 

 a specialised portion of the 

 chorion, which forms the 

 foetal part of the placenta, 

 and the foetus, but also 

 the remains of the yolk-sac, 

 and the duct by which it is 

 connected with the intestine 

 of the foetus. In animals 

 which develop outside the 

 mother's body (for instance, 

 birds) the yolk-sac is much 

 larger and is the great 

 source of nutriment during 

 growth. 



As the embryo grows, 

 the decidua capsularis is ex- 

 panded over its surface, and 

 as the growth continues the 

 uterine cavity is gradually obliterated, and the decidua capsularis is 

 forced into contact with the decidua vera, with which it fuses. 



As the decidua is merely thickened mucous membrane, it naturally 

 contains glands which become enlarged as the decidua thickens. It 

 was believed, at one time, that the villi of the chorion entered the 

 glands, but this is now known to be incorrect. The villi enter the 

 interglandular substance, and, in the human subject, the glands of the 

 decidua capsularis eventually disappear entirely. In the decidua 

 basalis and the decidua vera the superficial portions of the glands 

 also disappear; their deep portions remain in an almost unchanged 

 condition, and furnish the epithelium for -the regeneration of the 

 glands and the lining of the uterine cavity after parturition. The 

 intermediate parts of the glands in the decidua vera and the decidua 

 basalis become very much enlarged, and form a stratum of the 

 decidua called the spongy layer, and ultimately this layer is converted 

 into a series of clefts, and it is along the line of these clefts that the 

 decidua is separated at birth, 



FIG. 574. Diagram representing a later stage of develop- 

 ment than that shown in fig. 573. 1, Uterine muscle ; 

 2, villi of chorion of embryo ; 3, coalom ; 4, decidua 

 basalis ; 5, decidua capsularis ; 6, decidua vera ; 7, cavity 

 of uterus ; 8, allantois ; 9, amnion cavity ; 10, primitive 

 intestine ; 11, yolk-sac. 



